1. Field of the Invention
The present invention relates to an optical transmitter for outputting light signals to an optical transmission line and an optical communication system provided therewith.
2. Related Background Art
The common optical communication systems are provided with an optical transmitter for outputting light signals, an optical transmission line, such as an optical fiber or the like, for transmitting the light signals, and an optical receiver for receiving the light signals having propagated through this optical transmission line. It is generally known as to the optical transmission lines such as the optical fibers that the nonlinear optical, phenomena become easier to occur in the optical transmission line when the power of light signals propagating therethrough is high. It is also known that the nonlinear optical phenomena degrade the signal waveform, so as to tend to cause reception errors at the optical receiver. For this reason, it is an important subject in the field of the optical communication systems to restrain occurrence of the nonlinear optical phenomena and thereby prevent the degradation of the signal waveform.
There are a variety of nonlinear optical phenomena known as those occurring in the optical fibers, one of which is the stimulated Brillouin scattering (which will be referred hereinafter as SBS). For example, Reference 1: Y. K. Park et al., “A 5 Gb/s Repeaterless Transmission System Using Erbium-Doped Fiber Amplifiers,” IEEE Photon. Technol. Lett., Vol. 5, No. 1, pp. 79–82 (1993) describes the result of experiments to investigate the relationship between occurrence of SBS and degradation of transmission characteristics.
As techniques for restraining the occurrence of SBS there are, for example, the techniques described in Reference 2: S. K. Korotky et al., “Efficient Phase Modulation Scheme for Suppressing Stimulated Brillouin Scattering,” IOOC '95, WD2-1 (1995), Reference 3: D. A. Fishman et al., “Degradations Due to Stimulated Brillouin Scattering in Multigigabit Intensity-Modulated Fiber-Optic System,” J. Lightwave Technol., Vol. 11, No. 11, pp. 1721–1728 (1993), Reference 4: T. Sugie, “Transmission Limitations of CPFSK Coherent Lightwave Systems Due to Stimulated Brillouin Scattering in Optical Fiber,” J. Lightwave Technol., Vol. 9, No. 9, pp. 1145–1155 (1991), and Reference 5: L. Eskildsen et al., “Stimulated Brillouin scattering suppression with low residual AM using a novel temperature wavelength-dithered DFB laser diode,” Electron. Lett., Vol. 32, No. 15, pp. 1387–1388 (1996), and so on.
The SBS suppression techniques described in these References 2 to 5 are fundamentally the technology for suppressing the occurrence of SBS by phase modulation or frequency modulation of laser light emitted from a semiconductor laser as a light source. The phase-modulated or frequency-modulated laser light is outputted as light including signals to the optical transmission line after high-frequency signal components are superimposed thereon. The SBS suppression technique described in above Reference 5 is the technology of adequately reducing amplitude modulation of the laser light emitted from the semiconductor laser source by minimizing amplitude modulation caused during the phase modulation. In the semiconductor laser source, the phase modulation or frequency modulation of the laser light is effected by varying the refractive index of an active layer according to bias or driving current modulated by a modulation signal source, and thus it also brings about unintended amplitude modulation at the same time. Therefore, Reference 5 discloses the semiconductor laser source provided with special structure for suppressing the amplitude modulation caused during the phase modulation.